The present invention relates generally to piezoelectric transducers and sensors that measure acceleration, force, and pressure, and more specifically to a new method and structure for radially preloading an annular piezoelectric element of an accelerometer, force transducer or sensor, or pressure transducer or sensor.
Piezoelectric transducer and sensor designs have progressed significantly in recent years and have evolved from those which use natural piezoelectric crystals into designs which incorporate man made and electrically polled ceramic materials. Natural crystals are cut along specific planes and the resulting plate produces an electrical output when stressed either compressively or in shear as dictated by the particular crystallographic orientation of the cut. Incorporating shear cut plates into designs yields a transducer or sensor which is extremely insensitive to strains typically induced by base bending or thermal gradient exposure.
An example of a planar shear design accelerometer is shown in FIG. 1 and the sensing material can be either natural crystal or piezoelectric ceramic 12. A compressive stress is imposed on the sensing element by the tightened preload bolt 16, 18 and frictional forces are then present at all interfaces. Acceleration normal to the mounting base 10 imposes a shear stress on the sensing element and an electrical output is generated. The seismic system constructed from the two end masses 14 and center preload bolt 16, 18 is essentially isolated from any base distortion.
The man made ceramic piezoelectric elements can be fabricated in a variety of shapes and a tube form has been incorporated into many accelerometer designs. Tubes are electrically poled similar to ceramic shear plates and an ideally symmetric seismic system can be achieved. The components within this annular shear type accelerometer are relatively inexpensive to manufacture due to their simple, round, shape. An example of an annular shear accelerometer is shown in FIGS. 2 and 3. The support 20 is cylindrical and receives the cylindrical sensing element 22 and mass 24.
Attachment of the tube to the central mounting post is accomplished using a variety of methods. Epoxy attachment is very common but unreliable. Another approach uses ultra precise mated parts which are assembled after the post is cooled and the ring/mass is heated. When equilibrium returns, a preloaded assembly is achieved. This approach provides an excellent mechanical preload but is limited to a restricted usable temperature range. This approach also adds additional cost to accelerometer fabrication. An example is illustrated in U.S. Pat. No. 4,503,351.
The present transducer includes a center post extending along the sensing axis of the transducer. A piezoelectric sensing element concentrically engages the center post and extends along the axis. A mass concentrically engages the piezoelectric element and also extends along the axis. At least a first preload element preloads the piezoelectric element radially to the axis. In one embodiment, the center post includes a recess along the axis and the first preload element is in the recess and preloads the piezoelectric element radially outward.
The preload element may be a cylindrical spring. Also, the preload element in the recess of the center support may also be a solid element pressed fit into the recess to create the radially outward preload or may be a tapered element adjustably positioned into the recess to adjustably create the radial preload. Preferably, the center support of the piezoelectric element and the mass are cylindrical. Other shapes may be used.
The center support may extend from a base and an outer support spaced from the center support may also extend from the base and form therewith an annular recess in which the piezoelectric element and mass lie. Also, the mass may include a radial flange extending radially inward and over the piezoelectric element. Preferably, the piezoelectric element is a shear piezoelectric element which has a shear axis parallel to the accelerometer axis.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.